There are two types of organic electroluminescent (EL) display devices, passive (simple matrix) organic EL and active matrix organic EL. Organic EL materials are divided into low molecular weight organic EL materials and high molecular weight (polymer) organic EL materials. Low molecular weight organic EL materials are mainly applied by evaporation to the substrate. High molecular weight or polymer organic EL materials are applied by ink jet printing or similar techniques.
The active matrix drive method works for all known types of light-emitting diodes (LED) whose emission electroluminescent layer comprises a film of organic compounds. Examples of different types are small-molecule OLED (SMOLED), polymer light-emitting diodes (PLED) and phosphorescent OLED (PHOLED).
Small molecule organic EL substances are disclosed in U.S. Pat. Nos. 4,720,432 (VanSlyke et al.), 4,769,292 (Tang et al.), 5,151,629 (VanSlyke), 5,409,783 (Tang et al.), 5,645,948 (Shi et al.), 5,683,823 (Shi et al.), 5,755,999 (Shi et al.), 5,908,581 (Chen et al.), 5,935,720 (Chen et al.), 6,020,078 (Chen et al.), 6,069,442 (Hung et al.), 6,348,359 (VanSlyke et al.), and 6,720,090 (Young et al.), all incorporated herein by reference.
Large molecule or polymeric OLED substances are disclosed in U.S. Pat. Nos. 5,247,190 (Friend et al.), 5,399,502 (Friend et al.), 5,540,999 (Yamamoto et al.), 5,900,327 (Pei et al.), 5,804,836 (Heeger et al.), 5,807,627 (Friend et al.), 6,361,885 (Chou), and 6,670,645 (Grushin et al.), all incorporated herein by reference. The polymer light-emitting devices may be called PLED.
Organic luminescent substances also include OLEDs doped with phosphorescent compounds as disclosed in U.S. Pat. No. 6,303,238 (Thompson et al.), incorporated herein by reference. Organic photoluminescent substances are also disclosed in U.S. Patent Application Publication Nos. 2002/0101151 (Choi et al.), 2002/0063525 (Choi et al.), 2003/0003225 (Choi et al.), and 2003/0052596 (Yi et al.); U.S. Pat. Nos. 6,610,554 (Yi et al.) and 6,692,326 (Choi et al.); and International Publications WO 02/104077 and WO 03/046649, all incorporated herein by reference.
Active matrix organic LED devices are difficult to drive using simple two-terminal schemes because of their lack of memory. The rise and decay time of an organic LED device is very fast and it does not have intrinsic memory. To overcome this problem, thin-film-transistor (TFT) circuits have been developed to drive organic LED devices. Such circuits include four or more TFTs, a storage capacitor, and an organic LED pad arranged on a substrate. The storage capacitor enables the excitation power to an addressed organic LED element to stay on once it is selected.
While successfully overcoming the above mentioned problem, new problems in manufacturing are created. The storage capacitor process and deposition are very complicated and difficult to achieve in a fabrication process. The TFTs fabrication requires several mask steps whose difficulty and cost increase dynamically as the display size increases. If the substrate is plastic an expensive laser annealing process is used in fabrication of the TFT.